The arrival of the age of broadband transmission has been accompanied by demands for optical transmission systems with ever-greater capacities. It is becoming comparatively easier to achieve large capacities through wavelength division multiplex (WDM) technology, but there has also been much study of measures to raise the bit rate per wavelength. This is because by raising the bit rate per wavelength, device costs can be reduced, device sizes can be decreased, and power consumption can be lowered, enabling reductions in the initial costs and running costs for overall systems.
Electrical circuits which realize 40 Gbit/s/CH have already reached the stage of commercialization. In the WDM transmission of such high-speed optical signals, limitations on the distance of transmission due to chromatic dispersion, limits to the power input to fibers arising from fiber nonlinearity, and other problems arise. In particular, in recent years there has been much study of Differential Phase Shift Keying-Direct Detection (DPSK-DD) schemes as a means of coping with fiber nonlinearity.
Further, WDM transmission technology employing RZ (Return-to-Zero)-DPSK schemes and CS (Carrier Suppressed) RZ-DPSK schemes, which are still more resistant to nonlinearity, is also being studied. It is said that compared with the NRZ codes (Non-Return-to-Zero codes) used in conventional optical transmission systems, RZ codes are better able to accommodate input power limitations.
In a receiver employing DPSK-DD schemes (including RZ-DPSK, CSRZ-DPSK, and other RZ-type DPSK-DD schemes), a photodetector is used for direct detection after conversion into intensity-modulated codes of phase-modulated signals, using a demodulator such as Mach-Zehnder interferometer or similar. At this time, by using a double-balanced receiver, differential photo-detection is possible, and the discrimination sensitivity is improved by 3 dB compared with cases in which intensity-modulated signals are directly detected using a single photodetector; hence double-balanced receivers are generally used as the photodetector.
In order to use a Mach-Zehnder interferometer to demodulate phase-modulated signals into intensity-modulated signals, the path difference between the two paths of the Mach-Zehnder interferometer must be controlled at the wavelength level to follow fluctuations in the signal light wavelength. Methods to execute this control include, for example, a method in which the output level of the balanced photodetector is detected and a phase shifter provided in one arm of the interferometer is controlled such that a constant output level is obtained, as explained in Patent Reference 1.
As a Mach-Zehnder interferometer, an optical waveguide-type device fabricated on a PLC (Planar Lightwave Circuit) is marketed commercially. As the method of control of the path difference, it is possible to either control the substrate temperature (amount of change in pass band: 1.4 GHz/° C.), or to execute control by heating using a heater provided on both arms (amount of phase change: 1.33 π/W).    Patent Reference 1: Japanese Unexamined Patent Application, First Publication No. S63-52530